Question

Use the identity cos(alpha-beta)=cos(alpha)cos(beta)+sin(alpha)sin(beta) to find cos(5pi/12)

Answer 1

Wouldn't that be

cos(alpha-beta)=cos(alpha)cos(beta)+sin(alpha)sin(beta), and not

cos(alpha-beta)=cos(alpha)cos(beta)+sin(alpha)+sin(beta)?

Answer 2

Yes it would be

Answer 3

Think of two angles whose difference is 5pi/12, and one of which is an angle whose sine and cosine are easy to remember/evaluate.

Answer 4

11pi-6pi??

Answer 5

Divide both of those by 12, so that the resulting difference will be 5pi/12.